IRAS 15250+3609: minor merger revealed as an OH MegaMaser

ABSTRACT High-resolution European VLBI Network–Multi-Element Radio Linked Interferometer Network (EVN–MERLIN) observations have been conducted for the study of the nature and structure of the OH MegaMaser emission in the ultraluminous infrared galaxy (ULIRG) IRAS 15250+3609. The OH MegaMaser emission in IRAS 15250+3609 is found to be superposed on and confined within the nuclear source of the host galaxy having an active galactic nucleus (AGN) and an extended starburst region size of 130 by 95 pc. The 1667 and 1665 MHz OH emission covers a velocity range from –250 km s$^{-1}$ below to 400 km s$^{-1}$ above the assumed systemic velocity of the host galaxy with a strong and systematic velocity gradient from west to east across the nucleus. The excitation for any systemic and dominant foreground OH molecules comes from the far-infrared (FIR) radiation field emanating from the starburst nucleus of the host galaxy allowing for maser amplification of the nuclear radio continuum in the background. The velocity signature and the structure of the OH emission indicate that the masering foreground material is part of a progressing minor merger with a dwarf galaxy falling nearly radially into the nucleus of IRAS 15250+3609 from the west. Three velocity ranges can be identified in the merger process: high velocity for infalling material that is still mostly unperturbed, an intermediate velocity range where infalling material first interacts with the host galaxy, and a negative velocity region where material has already crashed into the host.